Some multi-band or other tactical radios operate in the high frequency (HF), very high frequency (VHF) (for satellite communications), and ultra high frequency (UHF) bands. The range of these multi-band tactical radios can operate over about 2.0 through about 512 MHz frequency range. Next generation radios will probably cover about 2.0 to about 2,000 MHz (or higher) to accommodate high data rate waveforms and less crowded frequency bands. This high frequency transmit mode is governed by standards such as MIL-STD-188-141B, while data modulation/demodulation is governed by standards such as MIL-STD-188-110B, the disclosures which are incorporated by reference in their entirety.
UHF standards, on the other hand, provide different challenges over the 225 to about 512 MHz frequency range, including short-haul, line-of-sight (LOS) communication and satellite communications (SATCOM) and cable. This type of propagation can be obtained through different weather conditions, foliage and other obstacles making UHF SATCOM an indispensable communications medium for many agencies. Different directional antennas can be used to improve antenna gain and improve data rates on the transmit and receive links. This type of communication is typically governed in one example by MIL-STD-188-181B, the disclosure which is incorporated by reference in its entirety. This standard specifies a family of constant and non-constant amplitude waveforms for use over satellite links.
The joint tactical radio system (JTRS) implements some of these standards and has different designs that use oscillators, mixers, switchers, splitters, combiners and power amplifier devices to cover different frequency ranges. The modulation schemes used for these types of systems can occupy a fixed bandwidth channel at a fixed carrier frequency or can be frequency-hopped. These systems usually utilize memoryless modulations, such as a phase shift keying (PSK), amplitude shift keying (ASK), frequency shift keying (FSK), quadrature amplitude modulation (QAM), or modulations with memory such as continuous phase modulation (CPM) and combine them with a convolutional or other type of forward error correction code.
These systems often use a number of base station segments that are operative with HF and VHF communications nets and often ad-hoc communications networks in which a plurality of N mobile radios are located on a terrain, typically each moving with no fixed infrastructure. Many of these systems must sample and downconvert a frequency or phase modulated intermediate frequency (IF) signal. Some of the signals are amplitude modulated waveforms with a combination of phase/frequency and amplitude modulations. These small and low power radios typically minimize the amount of circuitry used. One approach is to convert as quickly as possible to the digital or software domain where higher levels of integration exist. Software and firmware processing can be efficiently done at baseband and an efficient method to convert analog IF signals to baseband frequencies is required.
In a prior art downconverter system 10 such as shown in FIG. 1, IF signals are sampled at a sample rate (F) using an analog/digital converter 12 to create an N-bit sample signal. Within the digital domain illustrated at 14, a complex local oscillator circuit multiplies the signal and converts it into a complex baseband signal using the local oscillator 16 and mixer 18. Complex low pass filtering and downconversion follow using a low pass filter 20 followed by downconversion decimator 22, another low pass filter 24, and decimator 26 as illustrated. These functions can consume considerable digital resources and consume considerable power depending on their clock rates.